Underwater cables, in particular underwater optical fibre cables, are used for undersea communications. The underwater optical fibre cables are laid on the seabed and have landing points.
U.S. Pat. No. 4,389,088 relates to an underwater optical fibre cable including a central filament provided with helical grooves for receiving at least one optical fibre in each groove, the groove cross-section being such that an optical fibre therein can move transversally by at least half of its transversal dimension, wherein said central filament is made of a metal or a metal alloy. The central filament has an aluminium tube, applied by extrusion or by longitudinally welding a tape followed by drawing through a die. The function of such a tube is to protect the optical fibres from the effects of the outside pressure. In its turn, the tube is covered by an extrusion technique by a covering which is several millimeters thick and made of a plastics material such as polyethylene or polypropylene, so as to provide electrical insulation with respect to the outside medium. A layer of steel wires helically wound in contiguous turns surrounds the covering. The aim of such wires is simultaneously to absorb the tensile forces exerted on the cable and to form armouring capable of withstanding compression which results from outside pressure. The cable may optionally include another metal protection layer.
The Applicant observed that a cable according to U.S. Pat. No. 4,389,088, when is installed on the seabed in saltwater, it is subject to extreme environment which is corrosive for the components of the cable, especially for the metallic components.
The corrosion of the metallic components of the cable is dangerous because reduces the mechanical resistance of the cable and the water may finally reach the optical fibres. Moreover in acidic and neutral waters the cathodic reaction that generates the corrosion produces hydrogen that induces attenuation at the operating wavelengths of the optical fibres.
From the Norscan Report (August 2008, volume 2, issue 2) it is known that, in order to protect the armour of an optical fibre cable from a corrosion effect, a negative DC Voltage (Direct Current Voltage) linked to the cable armour is applied. When exposed armour areas of the cable come in contact with local ground, elements in the surrounding installation area will be attracted to the negatively charged exposed cable area. These elements will build up over time, forming a protective layer on the exposed cable armour as long as the negative DC potential is energizing the cable, thus slowing the corrosion process of the exposed armour area.
The Applicant has observed that the impressed current can cause hydrogen generation in the cable.
According to EP136877 a submarine cable comprises a cable core which is arranged within a protective sheath and provided with one or more layers of metallic armour. The armour is usually covered with a corrosion-protective layer of jute and/or polypropylene yarn impregnated with asphalt and/or bitumen. When the armour consists of steel wires or bands the steel is usually galvanised. Other metals are, however, also been used as cable armour. In connection with single conductor power cables, the conductivity of the steel armour has been improved by introducing wires of different metals into the armour.
This submarine cable is characterised in that integrated into the armour there are arranged wires, bands or tapes of a metal or metal alloy which is less precious than that of the armour, and which will represent a sacrificial anode for corrosion protection of the armour.